Featured image in the blog posts

[ludstuen90]

Allow users to add a featured image inside blog posts. This should allow users to specify one image, and have it be included as a featured image within posts. The image should take advantage of our site's existing style guidelines, and not require any additional sizing specifications from the user at the time of adding the featured image.

This featured image should also include alternate text, which can be displayed in situations where the image cannot.

Additionally, this image should show up as an image thumbnail in social media shares.